Travis CI: non-empty NODE_WEBKIT="0.8.3" is set for Node v0.10

There's currently no API to determine the most recent version of node-webkit over the Web.
install nw-gyp in 32 bit environment if NODE_WEBKIT is not empty

Note: `make clean` happens after `npm install nw-gyp -g` because we intend to use `nw-gyp` later to rebuild the existing build of node-sqlite3.
Travis CI: rebuild for the node-webkit (version `$NODE_WEBKIT`)
build for node-webkit 64bit on Linux (uninstall nw-gyp later)
attempt a build for node-webkit 0.8.4 (instead of 0.8.3)

Release notes https://groups.google.com/d/msg/node-webkit/bPjur7aFaQc/4TP6o3SM-8sJ say that 0.8.4 fix improves the behaviour on OS X.
do not uninstall nw-gyp on 64 bit Linux

It seems (in installation logs) that nw-gyp does not contain any binary (i.e. compiled) addons.

Thus it's fine if nw-gyp survives 64bit→32bit switch of our Node.js environment on Linux.
node-pre-gyp package testpackage (if NODE_WEBKIT is not empty)

This commit does also prepend `on Linux 64 bit:` to some comments (making it easier to understand what subplatform is targeted).

On Linux nw-gyp is installed earlier (in 64 bit mode) and thus we can refrain from its reinstallation for 32 bit, speeding things up.
added `--runtime=node-webkit --overwrite` twice

Try a feature originally introduced in node-pre-gyp v0.4.2 commit springmeyer/node-pre-gyp@370024a1a3a81c9faa35bfc2f030685c305bedca for springmeyer/node-pre-gyp#14.
oops, the previous commit requires node-pre-gyp v0.4.2 or newer
use the tarball version of node-pre-gyp v0.4.2

Working around a temporal inavailability of node-pre-gyp v0.4.2.

See springmeyer/node-pre-gyp#35 for details.
use `https` (instead of `http`) in the value of `remote_uri`

Otherwise the following check fails:

https://github.com/springmeyer/node-pre-gyp/blob/3d4632a7333d420d6423299b143f5391b626e904/lib/util/versioning.js#L75-79
try node-webkit's filename fix

A pull request (springmeyer/node-pre-gyp#36) is opened to land a partial fix for node-pre-gyp's validation process.

This commit uses a source branch of that pull request.

After this commit, node-pre-gyp's validator is expected to launch `nw` instead of `node-webkit` on Travis CI's Linux.

A former ENOENT error is expected to be replaced by a node-webkit's error when the engine realises it's given a node-sqlite3 module's JavaScript instead of some HTML5 application.
update node-pre-gyp's source branch

Commit Mithgol/node-pre-gyp@eaea6e159c9a052609cf93f0438cae17900f583b is now included in the node-pre-gyp's source used here.
on Linux 64 bit: download, unpack, use and erase node-webkit
for testing node-webkit, launch a virtual display

xvfb recipe:
http://docs.travis-ci.com/user/gui-and-headless-browsers/#Using-xvfb-to-Run-Tests-That-Require-GUI-(e.g.-a-Web-browser)

sleep 3 recipe:
http://docs.travis-ci.com/user/gui-and-headless-browsers/#Starting-a-Web-Server
on Linux 32 bit: download, unpack, use and erase node-webkit
also install libx11-6
try installing libx11-dev
sudo apt-get update --fix-missing

More magic.
save and restore PATH